Where a hydraulic motor is used to drive a traveling body, i. e., a hydraulically driven traveling vehicle, such a traveling purpose hydraulic motor is supplied with a pressure fluid. There, one of the parts of the hydraulic motor is supplied with a discharge pressure fluid from a hydraulic pump where the other or one port thereof is in fluid communication with a tank, to cause the hydraulic motor to be rotationally driven in one rotary direction or the other and then to cause a driving hydraulic fluid to be fed to the vehicle to drive the latter.
By the way, where a hydraulically driven traveling vehicle is traveling downhill, it can happen that the vehicle body may be driven to travel by the weight of the traveling vehicle itself to cause the hydraulic motor to be driven reversely by the vehicle body. Such a situation in which the feed of the driving hydraulic fluid is reversed, if continued, is highly dangerous since the hydraulically driven vehicle will then be moved downhill in an accelerated fashion.
Accordingly, there has been proposed, as disclosed, e. g., in Japanese Unexamined Utility Model Publication No. Hei 4-133003 and Japanese Unexamined Patent Publication No. Hei No. 4-50507, a braking valve called a counterbalance valve which can be switched so that the hydraulic motor may not be rotationally driven when it is reversely driven by an external force such as the weight of the vehicle body itself.
This can be illustrated with reference to FIG. 1 of the drawings attached hereto, which shows a drive circuit for a hydraulic motor, as disclosed in these publications. The drive circuit comprises a hydraulic pump 1, a first and a second main circuit 3 and 4 which are connected to the hydraulic pump 1, and a directional control valve 2 disposed between the hydraulic pump 1 and the first and second main circuits 3, 4 for supplying a discharge pressure fluid from the hydraulic pump 1 to the first and second main circuits 3 and 4 which are connected via a first and a second check valve 5 and 6 to a first and a second port 8 and 9 of a hydraulic motor 7, respectively. The drive circuit for the latter is further provided between the first and second main circuits 3 and 4 with a braking valve 11 that is designed to selectively connect and block a return circuit 12 through which a return fluid from the hydraulic motor 7 is passed to communicate with a tank 10.
And, with the directional control valve 2 at its first position a, if the first main circuit 3 is supplied with the pressure fluid from the hydraulic pump 1, the pressure built up therein (i. e., the driving pressure for the hydraulic motor 7) will cause the braking valve 11 to take its first position b to allow the second port 9 of the hydraulic motor 7 to communicate via the return circuit 12, the braking valve 11 and the directional control valve 2 with the tank 10. A return fluid will thereby be allowed to flow out of the hydraulic motor 7 into the tank 10. Thus, the hydraulic motor 7 is permitted to be rotationally driven in a given direction (as shown by the arrow c) to drive the traveling body.
In the state described above, while the vehicle is traveling downhill, if the hydraulic motor 7 tends to be reversely driven by the vehicle body to rotate in the direction of the arrow c in a runaway manner, it will be caused to undergo a pumping action whereby its first port 8 side has a reduced pressure and its second port 9 side has an elevated pressure. As a result, the first main circuit 3 will be reduced in pressure to cause the braking valve 11 to assume its neutral position e by a pair of springs 13. This will cause the return circuit 12 to be blocked and in turn the hydraulic motor 7 to be braked so as to be stopped.
It should be noted at this point that the case will equally apply where the pressure fluid is supplied into the second main circuit 4 so that the hydraulic motor 7 may be rotationally driven in the direction (indicated by the arrow d) that is opposite to the above described direction.
When such a construction is adopted, the return fluid from the hydraulic motor 7 is allowed to flow via the braking valve 11 and the directional control valve 2, pressure losses will thereby be created in the return fluid while flowing both through the braking valve 11 and through the direction control valve 2 and they will become increasingly greater to the extent to which the pressure at the low pressure side port (i. e., the second port 9 or the first port 8) of the hydraulic motor 7 may be elevated and its pressure difference with the high pressure side port (i. e., the first port 8 or the second port 9) may be reduced. As a consequence, the efficiency of driving the hydraulic motor 7 will be reduced.
Also, for this reason the braking valve 11 must have an increased area of opening through which the return fluid from the hydraulic motor 7 is allowed to flow. This will make it necessary for the braking valve 11 to be increased in size and for its space of installation to be enlarged.
Accordingly, with the above described problems taken into consideration, it is an object of the present invention to provide an improved hydraulic motor driving apparatus which enables a hydraulic motor to be driven at an enhanced efficiency and which allows it to be installed in a reduced space.